1. Field of the Invention
The instant invention relates to ceramic/organic multilayer interconnection boards. More specifically, the invention relates to printed wiring boards that utilize a novel construction of ceramic and organic layers to reduce difficulties commonly encountered by board materials and components attached thereto having differing thermal coefficients of expansion.
2. Description of the Prior Art
Multilayer boards have been utilized as a convenient and low cost means for mounting and interconnecting discreet electrical components. More specifically, multilayer boards, formed of a dielectric substrate, are provided with conductive metallic pathways which define electrical connections between discreet components mounted thereon. The metallic leads of the components may be soldered to the conductive pathways to complete the desired electrical connections.
The dielectric substrate used to form the printed circuit board is generally glass fiber cloth which has been impregnated with a resin formulation, such as epoxy or polyimide. The thermal coefficient of expansion of the dielectric substrate is significantly greater than that of the discreet components. The dissimilarity between the thermal expansion coefficient has not presented an insurmountable problem heretofore, since the flexibility of the metallic leads of the discrete components would compensate for the thermal mismatch. Even in situations where the circuit assemblies are subjected to frequent and great thermal changes or excursions, a manufacturer can compensate for the thermal mismatch by providing a circuit layout design which incorporates expansion loops and the component leads to absorb the varied expansion and contractions of the elements thereby preventing stress on the solder joints which is a major cause of circuit failure.
Recently, manufacturers have developed components housed in leadless chip carriers. The leadless component carrier housings are generally formed from alumina, and are directly affixed to a circuit board or to a conductive layer of a multilayer board. It has been determined that the difference in thermal coefficient of expansion between the dielectric substrate and the alumina leadless chip carrier, has resulted in a high degree of solder joint (used for attachment to multilayer boards) failures. More exactly, the thermal coefficient of expansion for a conventional epoxy glass multilayer board is in the range of 15-20.times.10.sup.-6 inch per inch per degree Celsius. In contrast, the alumina chip components have a much lower thermal coefficient of expansion, generally about 6.7.times.10.sup.-6 inch per inch per degree Celsius. Thus, when a circuit board laminate having leadless components is subjected to high thermal excursions (for example, in ranges greater than -55.degree. C. to +125.degree. C.), solder joints between the components and the laminate frequently failed since there was no flexibility between the components and the laminate to compensate for the varying amounts of expansion. The failures encountered were typically in the form of cracks in the solder joints.
Therefore, it is apparent that it would be desirable to provide a multilayer board having a thermal coefficient expansion which more closely matches the thermal coefficient of expansion of the alumina chip components to prevent failures of the desired electrical connections. Furthermore, it would be desirable to provide a multilayer circuit board which continues to utilize the organic glass epoxy substrates for the circuit board construction since the latter offers considerable cost advantages and are relatively easy to manufacture.
In addition, to problems of mismatched coefficients of expansion there are also thermal dissipation problems associated with the use of leadless components. Integrated circuits embodied in leadless components will not last long nor operate properly if they overheat. The heat rails heretofore used on standard multilayer boards to eliminate overheating problems are not capable of being used with leadless components because the leads therein come out from all four sides of the chip carrier leaving no pathway on standard multilayer boards for conventional heat rails.
From the foregoing, the need should be appreciated for a new and improved multilayer circuit board having both a thermal coefficient of expansion and a thermal dissipation capacity which are particularly useful in conjunction with leadless components. Accordingly, a fuller understanding of the invention may be obtained by referring to the Summary of the Invention, and the Detailed Description of the Preferred Embodiment, in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.